1. FIELD OF THE INVENTION
This invention relates to a process for producing benzophenone tetracarboxylic dianhydride (hereinafter referred to as BTDA) of low moisture absorbency, which is useful as a starting material for producing polyimides, polyamides and polyesters.
2. DESCRIPTION OF THE PRIOR ART
BTDA was used as a curing agent for epoxy resins, polyurethanes and the like. In recent years, it has gained in importance as a starting material for condensation polymers such as polyimides, polyamides, polyesters and the like. To be useful as a starting material for these polycondensates, BTDA is required to have an extremely high purity, because the polymerization degree of a polycondensate is greatly affected by the purity of monomers. Among the impurities of BTDA, especially harmful are compounds such as carboxylic acids, for example, acetic acid, propionic acid and benzoic acid, and carboxylic anhydrides other than BTDA, for example, acetic anhydride, phthalic anhydride and maleic anhydride, because these compounds act as an end-blocking agent to terminate the polycondensation reaction. When monomers containing a large amount of such impurities are used, the growth of polymer chain is generally hindered to a great extent and the molecular weight adjustment becomes difficult.
The presence of benzophenone tetracarboxylic acid (hereinafter referred to as BTA), which is the free acid of BTDA, is also undesirable, because it causes a decrease in the rate of polycondensation reaction and causes the resulting polymer to be inferior in properties.
BTDA is obtained by dehydrating the above BTA in the manner mentioned below to convert it into anhydride. Benzophenone tetracarboxylic monoanhydride (hereinafter referred to as BTMA) formed by dehydrating only two carboxyl groups as illustrated below also acts as an end-blocking agent in the polycondensation reaction owing to the low condensation reaction activity of the remaining carboxyl groups of these compounds under usual polycondensation conditions. ##STR1## When stored in the atmosphere, BTDA absorbs water to form gradually BTA and BTMA.
BTDA has been known to be produced by the dehydration of BTA as described above. The followings are known as specific procedures to carry out the reaction:
(1) Chemical dehydration by heating BTA in an excess of acetic anhydride [Acetic-anhydride dehydration method, as described, for example, in Zh. Org. Khim 4 (1) 163-168 (1968)].
(2) Dehydration by heating powdered BTA at 200.degree. C. or more under reduced pressure [Method of heating under reduced pressure, Visokomol Soedin, Ser. B 9 (1), 22-23 (1967)].
(3) Dehydration by fluidizing and heating powdered BTA with air heated at 200.degree. C. or more [Fluidizing and heating method, USSR Patent No. 420,609].
However, these known methods have the following problems:
In the case of (1) acetic acid dehydration method, the product BTDA inevitably contains harmful impurities such as the acetic anhydride used, acetic acid formed by the reaction of the acetic anhydride with the water removed from BTA, and the like, and the complete removal of said impurities was practically impossible.
In the case of both (2) method of heating under reduced pressure and (3) fluidizing and heating method, there is available no simple and reliable criterion for the complete conversion of BTA into BTDA, and hence, it follows that the reaction is frequently conducted at too high a temperature or for too long a period of time. Therefore, the resulting BTDA often has a cream to light brown color and the purity thereof is as low as 95 to 98% by weight.
Also, it is known that the BTDA obtained by the method of (1), (2) or (3) tends to absorb water from the atmosphere when stored in a plastic bottle, a fiber drum or the like, upon which the BTDA hydrolyzes into BTA or BTMA and the purity thereof decreases further with the lapse of time.